John N. Luff,
What Philately Teaches
“ Watermarks are designs impressed in the paper pulp. The paper is slightly thinner in the lines of these designs and appears lighter when held to the light. Of course you are all familiar with this appearance from having noticed 50the watermarks in note paper. On rare occasions the watermark is a thickening of the paper instead of a thinning. In such a case the watermark appears more opaque than the paper. Watermarks in paper used for stamps are, of course, intended as a security against counterfeiting. ”
